Electromagnetic security locks are well known in which an electromagnet is mounted to one of a door frame or a door and an armature is mounted to the other. In the predominant installations of such locks, the armature and electromagnet are generally mounted so that the face of the electromagnet and armature is parallel with a plane of the door and the holding force or attractions between the electromagnet and armature is perpendicular to the door when in a closed position. Such locks are known as surface locks. An attempted forced opening of the door is resisted by the electromagnetic attraction of the armature to the electromagnet.
This type of arrangement is very effective, providing a locking force against unauthorized opening of a door. However, as is generally the case, the armature must be mounted on the vertical surface of the door, while the electromagnet is mounted from the door frame and overhangs the top edge of the door. This type of lock, while very effective from a security standpoint, is not suitable for mounting on many types of swinging or double-acting doors.
Also, in many instances, for aesthetic purposes, a better concealment of the electromagnetic lock is desirable. This has led to increased popularity of the so-called shear lock, in which the electromagnet is mounted within the door frame and an armature is mounted at or adjacent the top edge of the door and is adapted to be attracted to the electromagnet when the door is in a closed position.
This type of magnetic shear lock presents some technical problems which must be considered. Typically, such a magnetic shear lock is mounted to a door and frame such that the electromagnet must exert an upward pull on the armature wherein the armature jumps the gap between the electromagnet and the armature to achieve locking of the door. The width of the gap which may be imposed between the electromagnet and the armature of conventional electromagnetic shear locks is limited by several factors. First, the electromagnetic field produced by the armature must be sufficiently great to bridge the gap and attract the armature. Second, the armature is generally biased against such movement so the door will positively unlock when the electromagnet is deenergized. Consequently, the magnetic field must overcome the force of gravity and the biasing force which oppose movement of the armature towards the electromagnet.
Commercial and design considerations limit the strength of the magnetic field which may be produced. To be commercially competitive, the overall design of the lock system should not impose unusual interface requirements or excessive installation costs. Consequently, the size of the electromagnet assembly is limited such that it will fit within the door frame and not require the removal of additional wall material. In addition, the electromagnet power requirements are limited such that they may be satisfied by standard power supplies and electrical circuits.
Therefore, the magnetic field strength, the orientation of the electromagnet and armature assemblies, and the biasing force limit the width of the gap in conventional electromagnet shear locks.
Accordingly, the present invention provides a new and improved electromagnetic shear lock with mechanical reinforcement which provides great resistance to attempted unauthorized entry, and is of reduced cost, and further provides a new and improved electromagnet mounting arrangement which allows a greater gap width.